We believe that considerable progress can and will be made towards the elucidation of the active site and the mechanism of action of the various carbonic anhydrases from both animal and plant origin. The structure of the human B and C enzymes (crystallized at pH 8.5) has been recently determined in Sweden to a resolution of 2 Argstrom units and the detailed parameters are now available. In our own laboratories we propose to extend our earlier work along the follow avenues: (1) The systematic study of the elementary steps and their combination in the overall mechanism of hydration-dehydration by carbonic anhydrase in both H2O and D2O. (2) The characterization of the factors responsible for the relatively low enzymic activity of carbonic anhydrase crystals (we have noted that the activity is more than 10 to the 3rd power times lower than that in solution). (3) Chemical modification of specific active site residues; recently we have shown that the simultaneous modification of both histidines found in the active site leads to complete loss of activity. We have also crystallized the modified enzyme with and without HCO3. (4) Further delineation of catalytic versatility; we have recently shown that CH3OCO2- is an important analog of HCO3- and that the enzyme also possesses triphospho esterase ("oxonase") activity. (5) The characterization of the binding properties of carbonic anhydrase towards colored analog substrates and inhibitors. (6) The characterization of the striking effect of phosphate and ribulose diphosphate ions on the binding of CO2 and HCO3- by spinach carbonic anhydrase (hexamer containing 6 Zn ion 2 per MW of 180,000), an effect which is completely absent in the erythrocyte enzyme. Clearly, this effect might clarify in part the regulatory role of carbonic anhydrase in the plant.